In the past it has been common practice to provide high voltage protection with communication cables or lines entering a power substation or generating plant in order to provide protection to personnel and equipment from ground potential rise and other hazardous high voltage fault conditions. Such protection has been necessary because the potential difference that can exist between two remote grounding systems can be very great during fault conditions, and this can lead to high voltages and currents being present and conducted on communication cables or lines connected to the station telephone, telephones, or other communication equipment during fault conditions.
Heretobefore such high voltage protection has been provided by interconnecting a neutralizing transformer between the station equipment and the line side of the communications system leading to a central communication or telephone exchange center. The neutralizing transformer is quite functional in such situations because the metallic path between the station side and line side is unbroken and DC signals and supervisory functions can conveniently be communicated between the station equipment and the central communications or telephone exchange center. Thus, because telephone communication systems employ DC signals such as DC dial pulse signals and "off-hook" DC signals, neutralizing transformers are widely used in telephone communication systems leading to power substations and generating plants and other sites where there is a real possibility of hazardous voltages being conducted on the cables or wires of the communication system leading to the station equipment or to the telephones connected on the station side.
While neutralizing transformers are functionally desirable because of their capability to communicate DC signals across the voltage protection barrier created thereby, the neutralizing transformer does have disadvantages and draw backs. In this regard, the neutralizing transformer is very expensive and quite heavy and bulky in design that makes its application and installation awkward and inconvenient. In addition, the nature of the design of the neutralizing transformer gives rise to a much greater risk of failure because the neutralizing transformer is completely dependent upon a good and positive remote ground that must be located a substantial distance from the station ground matt. If the neutralizing transformer is not properly and adequately grounded, then the high voltage protection supposely afforded thereby would fail during a fault condition. Moreover, the windings of the neutralizing transformer must be properly connected by polarity to effectively neutralize a high voltage potential since the fault current is directed into a separate winding to neutralize the fault current flow in the line windings. Thus, there is the risk of improper installation and connection and even the presence of the remote ground being corroded or otherwise developing a high resistance to current flow, or even the possibility that the remote ground might be connected to the station grounding. In all of the above cases, the neutralizing transformer would be ineffective to protect against a high potential rise or other hazardous voltage caused by a fault condition. It should be noted that most likely such conditions would go undetected until there is in fact a fault condition realized.
Isolation transformers have been interposed in such communications systems to provide a high voltage barrier within the communication system. Isolation transformers, as compared to neutralizing transformers, are less expensive, smaller, generally more reliable, easy to use and do not have the inherent problems discussed above that are associated with the neutralizing transformer. However, the nature of the isolation transformer results in the metallic path being broken between the windings of the transformer. While this is desirable from a safety consideration, it does present a problem insofar as transferring or communicating DC signals across the isolation transformer. In the past, various means such as high voltage reed relays and opto-electronic couplers have been employed to effectively transfer or simulate DC signals across an isolation transformer. However, such approaches have not been completely satisfactory and very successful as the transfer of simulated DC signals are often inaccurate, are not in exact time sequence with the source signal, or often distorted by the effect of the windings of the isolation transformer, and generally lack the precise control necessary to provide an effective and efficient DC signaling circuit for transferring DC signals and supervisory functions across such an isolation transformer. With isolation transformers, the inductance of the windings of the isolation transformer gives rise to a substantial amount of distortion in the communication system. This distortion when coupled with the natural distortion of the metallic communicating lines of the system seriously interferes with the logic content of any DC signals being communicated such as DC dial pulses from a telephone located on the station side of the isolation transformer. This distortion obviously leads to an ineffective communication system.